1. Field of the Invention
The present invention relates to surface emitting distributed feedback semiconductor lasers and, more particularly, to a surface emitting distributed feedback semiconductor laser array that uses a Talbot spatial filter to establish fundamental lateral mode oscillation.
2. Description of the Prior Art
Light amplification by the stimulated emission of radiation (laser) produces unidirectional, monochromatic, and most importantly coherent visible light. The stimulated emission of radiation is a process in which the energy state of an atom changes in a quantum transition with the emission of a photon. During such a process, a photon approaches an atom, initially in an excited energy state, and induces this atom to make a transition to a lower energy state. As the atom's energy state is lowered, the atom emits a photon. This emitted photon, which is separate from the photon that induced the energy transition, possesses an energy that is equal to the difference between the excited and the lower energy states of the atom. Moreover, this emitted photon and the inducing photon both leave the atom in the same direction the inducing photon had as it approached the atom. These exiting photons are also exactly in relative phase with one another; that is, they are coherent. This coherence is dictated by energy conservation in that if the two photons were out of phase by any amount they would interfere destructively, thereby violating energy conservation. Therefore, stimulated emission of radiation is a process that induces coherent photon multiplication or light amplification, thus a laser.
Laser technology has evolved by applying the above stated principle to several different types of active media. The most recent development in this field, coupled with the advancements in semiconductor fabrication technology, is the semiconductor laser. Unlike an atomic laser, however, stimulated emission in a semiconductor laser occurs when there is a solid state material in an excited state. Thus, stimulated emission in a semiconductor laser involves more than one atom.
A surface emitting distributed feedback semiconductor laser is a device that produces unidirectional, monochromatic, coherent visible light through stimulated emission in semiconductor materials. Such a device has a positively doped side and a negatively doped side that are joined at a junction, and a grating surface that is etched into an outer surface of the positively doped side. The grating surface, upon which a strong conductive material is deposited, provides a means by which coherent photon energy fields may be diffracted. A second order grating design permits deflections of coherent photon radiation to be directed normal to an output window etched into the negatively doped side of the junction through first order diffraction, and directed parallel to the grating surface through second order diffraction. The first order diffraction produces a beam of unidirectional, monochromatic, coherent visible light at the output window, whereas the second order diffraction provides a feedback of photon radiation to an active region that is adjacent and parallel to the grating surface.
A theoretical longitudinal mode near-field intensity profile produced at the output window of a surface emitting distributed feedback semiconductor laser device is antisymmetric with a zero intensity null at the output window center. A corresponding theoretical longitudinal mode far-field intensity profile is double-lobed and symmetric about the output window center. These theoretical intensity profiles have been practically demonstrated in actual single device measurements as described in the article Surface Emitting Distributed Feedback Semiconductor Laser, Applied Physics Letters, Volume 51, Number 7, pp. 472-474, August 1987.
The incorporation of a linearly varying chirp in the second order grating design, however, results in more desirable theoretical and practically demonstrated intensity profiles. For instance, the theoretical longitudinal mode near-field intensity profile produced at the output window of a chirped grating surface emitting distributed feedback semiconductor laser device is without an undesirable zero intensity null at the output window center. Furthermore, the corresponding theoretical longitudinal mode far-field intensity profile has a desirable single lobe. These more desirable theoretical intensity profiles have been practically demonstrated in actual single device measurements as described in the related and copending patent application and assigned to the assignee hereof Ser. No. 07/826,720, entitled, Chirped Grating Surface Emitting Distributed Feedback Semiconductor Laser, filed on Jan. 28, 1992. Such measurements have also demonstrated that a single chirped grating surface emitting distributed feedback semiconductor laser device is capable of producing a desirable 350 mW of continuous wave power in a single-lobed far-field at an efficiency of up to 20%. It can thus be inferred from the many desirable characteristics of a single chirped grating surface emitting distributed feedback semiconductor device that there are a wide variety of applications in which one or more of these devices can be used.